Great science and technology spoiled by exaggerated off-hand remark.

A lot of good science is driven by the availability of technology. The laser and nuclear magnetic resonance spectroscopy (MRI) have had an incredible impact on science, medicine, and Western society in general. One key stage in many of these technological developments has been a transition from something like a national facility (through an institute-level facility) to an in-house instrument that individual research groups have and use on a routine basis.

Accelerator facilities, which provide beams of high energy electrons and/or X-ray photons, are still at the national facility stage. A newaccelerator technology, however, is promising to change all that. In the not-so-distant future, every science department may have ready access to high-energy electrons and X-ray lasers right in their basement.

Currently, accelerators that can provide beams of electrons in the Giga-electronVolt (GeV) range are massive devices. For instance, the electrons that power the X-ray laser at Stanford are accelerated to around 17GeV over a distance of 1 km. Not only is this simply not feasible to build at every university, but many Western countries can't even afford one of these, so the few that exist end up serving scientists from around the world.

You'll notice that the example I used is an X-ray laser source. The only way to generate intense laser pulses in the X-ray region is by deliberately accelerating electrons in such a way that they generate coherent light. These devices, called free electron lasers, have been around for many years. But they are big devices that require their own staff to operate and maintain.

To get to do an experiment at a free electron laser facility, you have to put in a proposal. Both you and the scientific staff at the facility have to agree that the experiment will generate something useful and interesting. That makes applicants think very hard about what sort of experiments they want to do. Unfortunately, this means they tend to choose relatively safe experiments. There are no Friday night, do-it-to-see-what-happens experiments at free electron laser facilities.

This would change if it were possible to generate beams of high-energy electrons in the confines of your own basement. This is not an easy task, and the usual approach is to gently accelerate electrons up to the required energy over rather long distances. An alternative, however, is laser wakefield acceleration, which achieves very high energy in a very short distance.

Ride that electron

Laser wakefield acceleration is one of the coolest things in physics. And the best way to see it is to take a ride on one of the electrons. So there you are, meandering around your home atomic nucleus with your friends. The community is a bit spread out because you are part of a dilute gas in a vacuum chamber. But all is calm and happy.

In the distance, you observe a big, angry-looking light pulse that is rapidly approaching. Having experienced the disruption of a passing light field before, you brace yourself for a bit of a bouncy ride. However, this ride exceeds even your worst expectations. The light field grabs you and all your friends and shakes you so violently that you are thrown out of your comfy atomic home and into the vacuum. Then, without so much as an "oops, sorry," the light pulse moves on.

All those lovely bare nuclei just seem so attractive now—they should, being positively charged and you negatively charged—and there are so many of them, so close, just waiting to be neutralized. This sets up the equivalent of a massive downhill slope, with you and your electron-friends perched precariously at the top in a soapbox derby car that has no brakes. You accelerate down the potential hill toward the waiting nuclei.

But by the time you get there, the light pulse has created even more attractive-looking nuclei in the distance—you know the sort, with a jacuzzi and a bit of backyard to call your own. The hill steepens and seems without end, and there is no other direction but down. You race on down the potential hill, still accelerating.

You keep chasing the light pulse, driven by the cloud of electrons racing behind you and all those positive charges right in front of you. All of a sudden, the light pulse accelerates away and vanishes. Shortly afterward, you blow through the last of the charged nuclei and find yourself drifting in the vacuum at a speed that is very close to the speed of light, and a kinetic energy of around a GeV.

All of this happens over a distance of a just a centimeter or so, and over a few picoseconds. It is truly the electron's equivalent of being struck by a tornado and finding out that the promised basement is actually a third-floor balcony.

We need MOAR power

The latest development is a good news/bad news joke, accompanied by a terribly overblown conclusion. The good news is that scientists at the University of Texas in Austin have demonstrated that laser wakefield acceleration can generate electrons with energies over 1GeV. This was not possible previously.

To achieve this, though, required that the density of the gas used to provide the electrons was reduced by an order of magnitude. Unfortunately, as the gas density reduces, the required laser energy increases. So instead of using a Big Laser™, the researchers built a Really Big Laser™. To put it in perspective, a typical laser wakefield acceleration experiment uses a laser that produces something like ten laser pulses per second, with each pulse having about 1J of energy (average power flow during the pulse is about 10TW). And that is a complicated beast, involving one laser and around four amplifier stages, occupying two to three large tables.

The laser in this experiment emits a pulse once per hour, but each pulse has 120J of energy. The power flow in this system is around 1PW. I suspect that this system requires considerably more than three tabletops for the optical components, never mind all the support services.

In any case, the increased pulse energy provides electric fields sufficiently large to allow laser wakefield acceleration in low-density plasmas. Even better, their system hasn't reached its limit. Currently, the laser focus isn't that good, so even higher intensities—and therefore larger accelerating fields—can be reached with some optimization. Under optimum conditions, they should be able to reach 10GeV. At that energy, they are less than a factor of two short of the most energetic free electron lasers.

Unfortunately, at one shot per hour, that laser isn't going to be very useful. Keeping experimental conditions under control and repeatable for hours on end while you wait for the laser to cycle so you can get just a few measurement points sounds like a nightmare. To really make this thing useful, I would say that they need to get to something like a shot per second, with ten shots per second as a better target.

In the conclusions, though, lurked the kind of statement that gives scientists a bad name: "LPAs [laser-plasma accelerators] at these densities are required for tabletop X-ray free electron lasers and to achieve the ~10GeV stage envisioned for a future laser-driven collider." Calling this, or any like-system "tabletop" is like calling me a "nobel prize winning physicist"—the sort of exaggeration that causes the world to wobble on its axis. And of course, this was precisely the language that the press release (and many ensuing press reports) picked up and used.

Even if we accept that the laser system can be halved in size—something that seems challenging—this device will occupy at least two, and more likely three, heavily shielded rooms. Yes, the chamber where the electron bunch is accelerated is small; in that respect it is already a tabletop system. But really, even that small part requires a good vacuum system, gas injectors, supply, and diagnostics. All of which adds up to a considerable amount of space.

The point is that this doesn't have to be a tabletop device. Hell, most research laser systems are only tabletop if you ignore the large box sitting beside the table, so I don't get the obsession with the label. Even if this tech requires the accelerator to be in a separate building (or in the basement of a building), it would be a considerable improvement over a 1 km beam line. The exaggeration was not just unnecessary, but considering that most users are not experts in the field, it raises false expectations that could end up damaging the credibility of accelerator physicists.

Ref. 26 in the paper describes the PW laser. While that reference doesn't give an actual size for the thing, looking at the schematic it's probably a good-sized room filled with the necessary amplifier stages and so on.

Tabletop does seem to be hyperbole, but it's a big advance in itself that you can buy this hardware to install in an existing facility, rather than having to build the facility around the purpose of housing the laser.

Tabletop does seem to be hyperbole, but it's a big advance in itself that you can buy this hardware to install in an existing facility, rather than having to build the facility around the purpose of housing the laser.

Well, the point to the "tabletop" bit is that the accelerator itself really does fit on a tabletop. That alone is a significant advancement, and actually the notable bit about the whole thing (that they managed to perform the acceleration over a comparatively minuscule distance). Yes, it requires a powerful laser to work, but this was just an experiment to prove it can be done, not a test of a finished accelerator assembly.

Laser technology is constantly improving. It might take a laser the size of a large room to do this now, but in 10 years we might be able to do the same thing with something that can actually fit on a desktop. Lasers are something very many research facilities will already have or want for many reasons (albeit most don't have one on this scale quite yet), it's not like the laser is necessary for the accelerator and the accelerator alone (in contrast with the kilometer long assembles needed for previous methods).

No, this breakthrough doesn't mean in a few months you'll be able to fedex an X-ray laser to your lab, but this is the kind of breakthrough that could lead to exactly that.

I always thought something like this would make for an awesome roller coaster. Have it look like your coaster is going to collide with another on coming coaster then sharply veer away at the last minute. Depends would be on hand for riders.

Can you elaborate on how the electrons get accelerated? The light pulse liberates the electrons from successive nuclei in its path, correct? But on a gross scale the plasma is still electrically neutral. So how does there exist a gradient that causes acceleration in one direction?

Given the diminution of lasers over the last 2 decades it won't be long until the laser to drive this device is truly tabletop-sized.

Actually, the lasers which count (as far as this application is concerned) have not gotten any smaller in the last 20 years. The power supply has, perhaps, halved in size, but otherwise there has been very little change

Can you elaborate on how the electrons get accelerated? The light pulse liberates the electrons from successive nuclei in its path, correct? But on a gross scale the plasma is still electrically neutral. So how does there exist a gradient that causes acceleration in one direction?

Because in the process of creating the plasma you do not have charge neutrality of the width of the laser beam. The pondermotive force drives all the electrons to the outside, creating a bubble of positively charged material surrounded by a skin of electrons. The electrons are all accelerated towards the most positive part of the bubble. From the point of view of the electrons, the bubble is just behind the laser pulse, so it appears to move at the speed of light through the plasma (or close to it), so the electrons are always chasing that positive charge.

I haven't seen the news articles referenced, so maybe I don't completely understand what's going on here... but it sounds like this is a potential replacement for large national-scale particle accelerators? The kind that currently require *miles* of subterranean tunnels and equipment sometimes spanning multiple countries?

If that's the case, I think "tabletop" is a pretty reasonable description for a replacement that takes "2 or 3 heavily shielded rooms". Okay, maybe it's not perfectly literal, but this seems a little pedantic.

Article:You accelerate down the potential hill toward the waiting nuclei.

But by the time you get there, the light pulse has created even more attractive-looking nuclei in the distance—you know the sort, with a jacuzzi and a bit of backyard to call your own. The hill steepens and seems without end, and there is no other direction but down. You race on down the potential hill, still accelerating.

I don't get why, having reached a near nuclei, a distant nuclei should be more attractive. What makes the difference that pulls the electron away from the bird in the hand towards the ones out there in the bush?

To get to do an experiment at a free electron laser facility, you have to put in a proposal. Both you and the scientific staff at the facility have to agree that the experiment will generate something useful and interesting. That makes applicants think very hard about what sort of experiments they want to do. Unfortunately, this means they tend to choose relatively safe experiments. There are no Friday night, do-it-to-see-what-happens experiments at free electron laser facilities.

I really love that kinda articles - explaining complicated things that not-everyone-knows about in a way that curious people who are not physicist can understand. Also, shows that science doesn't have to be dead-serious all the time.

And as for the press-release exaggeration - that's why i tend to dislike journalism in general. People always go for the flashy titles, exaggeration and end up publishing bullshit. The other day i was "cited" in a local newspaper, and when i read the actual article i didn't quite remember having said the things they had written...

Oh well, at least some journalists keep it real, on some sites/medias...

Considering the fact that cyclotrons used for proton radiation therapy only produce particles in 100-300Mev range and take up most of a hospital basement, using the term "tabletop" isn't much of an exaggeration when one puts these things in perspective. Let's also not forget the fact that to produce energies in the Gev range normally will require superconducting magnets and all of the support infrastructure that goes along with that. So to say that the author is being pedantic about this is being charitable.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.